The demand for advanced electronic systems in e.g. front-end wireless communication and the increased use of the Internet has resulted in miniaturization of electronic products having more functional features. These products use a large number of passive components (e.g. resistors, capacitors and inductors). In conventional printed circuit boards, these passive components are surface mounted devices, which consume a large area thereby increasing costs. Passive components generally may take up to 60% of the printed circuit board (PCB) area thereby limiting the space available for active components (e.g. integrated circuits (IC))
On the other hand, passive components, such as bypass capacitors are generally placed as close as possible to a die or IC to increase the effectiveness of the capacitors. These capacitors and other passive components are thus often surface mounted to the die side or on the opposite side of an IC during printed circuit board assembly. FIG. 1 illustrates a cross-section of a conventional multi-layer printed circuit board assembly 10 having an IC 11, capacitors 12 located on the die side and capacitors 13 located on the opposite side of the die. The terminals of the capacitors 12, 13 are internally connected to the integrated circuit through pads 9, vias 14, and power or ground lines 15, 16.
Reducing the dimensions of the passive components may help in mounting a large number of such passive components. However, reducing the dimensions of the surface mounted passive components has an adverse effect on the electrical performance.
An alternative that is currently practiced for selected applications is embedded passive components using screen printed inks. Commonly used materials include capacitor paste and resistor inks. The density of capacitance that can be achieved using commercially available printed paste is insufficient to meet a wide range of capacitance requirements that is readily available using discrete capacitors. Printed resistor inks have poor resistance tolerance of up to 30%. In such instances, laser trimming may be used for precise resistance tolerance control for selected applications which increases costs.
Japanese Publication No. 2002261449 published on 13 Sep. 2002 in the names of SEIICHI et al. describes a method of embedding IC chips and discrete passive components by using a proprietary epoxy resin material. Interconnections between the ICs and passive components are formed by vias, which are composed of a conductive resin. The use of proprietary material and via forming process in commercial applications increases costs.
U.S. Pat. No. 6,407,929 B1 issued on 18 Jun. 2002 to AARON et al. relates to embedding discrete passive components by a lamination process. Interconnections are formed after the lamination process by drilling holes right above the component terminals and filling the holes with a conductive material.
United States Patent Publication No. 2004/0001324 A1 published on 1 Jan. 2004 in the names KWUN et al. describes a method of embedding IC chips in a through-hole cavity and discrete passive components in a PCB recess (partially formed cavity). A dielectric epoxy material is filled in the cavities, and once the epoxy is cured interconnections are formed by vias formed in the cured epoxy over the component terminals.
International (PCT) Publication No. 2004/001848 A1 pertains to a lamination process for embedding IC or discrete passive components in a PCB with cavity formed on prepreg sheets to fit the components. Interconnection are formed by drilling holes right above the component terminals and electroplating.
There are a number of concerns relating to the processes of embedding discrete passive components in the above documents. For instance, drilling holes for vias above the component terminals may introduce thermal/mechanical damage or removal of termination material. Electroplating the drilled vias over component's solder terminal may not be feasible due to concern of plating adhesion to solder surface.
It is with the knowledge the above concerns that the present invention has been conceived and is now reduced to practice.